In recent years, as electronic equipment becomes smaller and electric signals become faster, electronic parts used in electronic equipment are becoming smaller and multifunctional. Some examples of these small, multifunctional electronic parts are BGAs (Ball Grid Arrays), CSPs (Chip Size Packages), and MCMs (Multichip Modules), which are collectively BGAs. A BGA comprises a BGA substrate having s a large number of electrodes provided in a grid-like pattern on the rear surface thereof. When a BGA is mounted on a printed circuit board, the electrodes of the BGA substrate are bonded to the lands of the printed circuit board using solder. When mounting a BGA on a printed circuit board, if soldering is performed by individually supplying solder to each electrode, not only is a great deal of effort required, but it is not possible to supply solder from the exterior to electrodes located in a center portion of the BGA. Therefore, when mounting a BGA on a printed circuit board, a method is employed in which a mound of solder is previously placed on each electrode of the BGA. This process is referred to as solder bump formation.
Solder bumps are formed on a BGA using solder balls, solder paste, or the like. When solder bumps are formed using solder balls, a sticky flux is applied to the BGA electrodes, and then solder balls are placed on the flux-coated electrodes. The BGA substrate is then heated in a heating apparatus such as a reflow furnace to melt the solder balls and thereby form solder bumps on the electrodes. Semiconductor substrates such as BGA substrates and CSP substrates are collectively called module substrates. When solder bumps are formed on the lands of a wafer using a solder paste, a metal mask having punched holes of roughly the same size as the lands of the wafer in positions matching the lands is placed on the wafer, and a solder paste is spread and wiped with a squeegee from atop the metal mask to print the solder paste on the lands of the wafer. The wafer is then heated in a reflow furnace to melt the solder paste and thereby form solder bumps.
In the case of conventional BGAs, solder balls made of an Sn—Pb alloy were used to form solder bumps Solder balls made of an Sn—Pb solder alloy are not only superior with respect to solderability to BGA electrodes, but particularly an Sn—Pb alloy having a eutectic composition has a melting point that has no thermal effects on BGA elements, substrates, or the like at the time of soldering. In addition, since the solder balls contain Pb, which is soft, the resulting solder bumps can absorb impacts when an electronic part or electronic equipment in which these solder balls are used is dropped, and this ability greatly contributes to increasing the lifespan of electronic parts, electronic equipment, and the like. However, the use of Pb is now being increasingly regulated on a global scale, so naturally an Sn—Pb eutectic composition which has conventionally been used for soldering is also being regulated.
Conventionally, Sn—Ag—Cu based solder alloys such as Sn-3.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu have been used as compositions for lead-free solder balls for BGAs. These lead-free solder alloys have excellent heat cycle properties. However, when portable electronic equipment in which solder balls having these solder alloy compositions are used is dropped, interface peeling easily takes place at the solder ball bonding interface, and for this reason, these alloys have been thought to have poor resistance to drop impacts (resistance to impacts due to dropping).
The following solder alloy compositions for lead-free solder balls are proposed to prevent impacts due to dropping of portable electronic equipment: a lead-free solder alloy comprising, in mass %, (1) Ag: 0.8-2.0%, (2) Cu: 0.05-0.3%, and (3) one or more elements selected from In: at least 0.01% and less than 0.1%, Ni: 0.01-0.04%, Co: 0.01-0.05%, and Pt: 0.01-0.1%, and a remainder of Sn (WO 2006/129713 A, Patent Document 1); a lead-free solder alloy which is characterized by comprising Ag: 1.0-2.0 mass %, Cu: 0.3-1.5 mass %, and a remainder of Sn and unavoidable impurities and which may further contain one or more of Sb: 0.005-1.5 mass %, Zn: 0.05-1.5 mass %, Ni: 0.05-1.5 mass %, and Fe: 0.005-0.5 mass % with the total content of Sb, Zn, Ni, and Fe being at most 1.5 mass % (JP 2002-239780 A, Patent Document 2); a lead-free solder alloy comprising, in mass %, 0.1-1.5% of Ag, 0.5-0.75% of Cu, Ni in an amount satisfying the relationship 12.5≦Cu/Ni≦100, and a remainder of Sn and unavoidable impurities (WO 2007/081006 A, Patent Document 3); and a lead-free solder alloy comprising 1.0-2.0 mass % of Ag, 0.3-1.0 mass % of Cu, 0.005-0.10 mass % of Ni, and a remainder of Sn and unavoidable impurities (WO 2007/102588 A, Patent Document 4). In addition, a method of applying a flux to the electrodes of a module substrate is disclosed as a method of solving the problem of fusion defects which develop at the time of bonding a module such as a BGA substrate to a printed circuit board (WO 2006-134891 A, Patent Document 5).